Not sure I understand how a 911 is the next step from an M3 or any M for that matter? Unless you are simply referring to the more expensive aspect when you consider that to really out class an M3 with a porsche you need to go to a Turbo which is in the $160K range (new).

An M6/M5 is simply BMW's submission into the GT and Sports Saloon market. Similar to what Porsche did with the Panamera, or Maserati with a GranTurismo, or Jaguar with an XKR (S), etc.

Its a logical progression upwards...where do you go after an M3..another M3..meeh? maybe if the new one is as good as the old..but I will probably go for a 911, Ferrari or Lambo next